Examples of liquid surface position detection devices may include a device for detecting a liquid surface position using a float floating on a liquid surface in a tank, and a device in which an ultrasonic wave propagation body is disposed in a tank and a liquid surface position is detected using a difference between the speed of surface waves propagating through the liquid and the speed of surface waves propagating through the gas. Various types of liquid surface position detection devices using ultrasonic waves have been proposed (for example, see Patent Literature 1 (FIG. 1)).
In the liquid surface position detection device of Patent Literature 1, a metal ultrasonic wave propagation body is disposed in a tank so that the longitudinal direction of the propagation body coincides with the vertical direction, and an ultrasonic vibrator provided at an upper end portion of the ultrasonic wave propagation body is vibrated. A liquid surface position is detected using a phenomenon that the speed of surface waves propagating through a liquid contacting portion is slower than the speed of surface waves propagating through an exposed portion exposed from the liquid. However, since the ultrasonic wave propagation body is made of metal, the speed of surface waves propagating through the liquid contacting portion and the speed of surface waves propagating through the exposed portion are substantially the same, whereby detection accuracy of the liquid surface position is lowered. To address this issue, a technology using a propagation body made of synthetic resin is proposed (for example, see Patent Literature 2 (FIGS. 1 and 2)).
In the liquid surface position detection device of Patent Literature 2, the propagation body made of synthetic resin is disposed in a tank so that the longitudinal direction of the propagation body coincides with the vertical direction. A piezoelectric element provided on an upper surface of the propagation body is vibrated to generate surface waves on a surface of the propagation body. Time until the surface waves reflected at a lower end of the propagation body returns to the piezoelectric element is measured. A liquid surface position is detected using a phenomenon that the speed of surface waves propagating through a liquid contacting portion is slower than the speed of surface waves propagating through an exposed portion exposed from the liquid. The piezoelectric element is disposed to be pressed against the upper surface of the propagation body by a fixing member.
However, since the propagation body is disposed in the tank in the liquid surface position detection devices of Patent Literature 1 and Patent Literature 2, there is a possibility that the liquid in the tank comes into contact with the piezoelectric element. Therefore, a technology for preventing a liquid from coming into contact with a piezoelectric element in a liquid surface position detection device in which a piezoelectric element is disposed at an upper end portion of a propagation body and a liquid surface is detected using ultrasonic waves is required.